1. Field
The present disclosure relates to techniques for communicating optical signals. More specifically, the present disclosure relates to an optical module and an associated technique for edge-to-edge coupling of optical signals between chips.
2. Related Art
Many existing optical modules use on-chip optical waveguides to communicate optical signals. For example, in a proposed optical module chips are oriented face-to-face. In this configuration, optical signals may be optically coupled between the chips in a direction normal to the surface of the chips (i.e., in a direction out of the planes of the chips) using mirrors or grating couplers, which are positioned at the ends of the on-chip optical waveguides. Note that this optical coupling is sometimes referred to as ‘optical proximity communication.’ However, using face-up and face-down chips in this proposed optical module can significantly increase packaging complexity and cost, especially if the face-up and face-down chips are relatively large, and also if the chips include optical waveguides fabricated on silicon substrates using silicon-on-insulator technology.
An alternative to using face-up and face-down chips is to couple the optical signals between the chips via optical fibers. For example, in the plane of a chip, an optical fiber may be aligned with an on-chip optical waveguide by using a tapered or an inverse tapered optical fiber at the edge of the chip. In order to allow sharp turns in the routing of the optical waveguides on silicon substrates, these optical waveguides typically have modes with a small spatial extent (which are close to the diffraction limit). These modes are facilitated by the strong confinement associated with the large index of refraction difference between the waveguide core and surrounding cladding materials. However, because an optical beam diverges quickly as leaves such an optical waveguide (which is referred to as mode mismatch), it can be difficult to achieve low-loss, edge-to-edge optical coupling.
In some existing chips, the optical losses associated with mode mismatch can be alleviated by using mode converters between aligned optical waveguides. For example, a mode converter with a 3-dimensional mode taper or nano-taper at the end of an optical waveguide can convert a sub-micron waveguide mode into an optical-fiber-sized mode. In the process, the mode converters can increase the optical coupling distance at an edge of a chip by up to a few tens of micrometers without significant mode-mismatch loss.
However, mode converters increase the cost and complexity of chips. In addition, for the mode converters to effectively couple the optical signals between chips, the optical waveguides on these chips still need to be aligned. Furthermore, this alignment needs to be maintained, even in the presence of disturbances, such as: thermal expansion, vibration and shock. Consequently, direct coupling of optical signals between chips in optical modules remains challenging.
Hence, what is needed is an optical module without the above-described problems.